1. Field of the Invention
The present invention relates to a focus servo apparatus which performs a focus search operation of moving focus position of a light beam directed to a signal layer of a signal recording medium from an optical pickup along a focusing direction, thereby bringing the focus position into a focus servo area.
2. Description of the Related Art
For signal recording media such as optical discs which to optically reproduce and/or record a signal from and onto using laser light, new format optical disc standards such as the Blu-ray standard and the HD-DVD (High Density Digital Versatile Disc) standard have been proposed which are improved in recording density over DVD and CD that are current optical disc standards.
Due to the improvement in recording density, stricter requirements are imposed on optical characteristics of optical pickups for these optical disc standards having improved recording density in order to raise signal recording qualities, and an optical pickup is known which is provided with an aberration correction lens for correcting spherical aberration caused by its output optical system for directing laser light to an optical disc (refer to Japanese Patent Application Laid-Open Publication No. 2003-257069).
The aberration correction lens is constituted by a beam expander or a collimator lens, which is displaceable along the optical axis direction and can be driven by an actuator, and is displaced so as to minimize spherical aberration that occurs in laser light focused on a signal layer of an optical disc through a cover layer (a transparent substrate) covering the signal layer.
In the case of the Blu-ray standard, NA of the objective lens is 0.85, which is much greater than 0.6 for DVD and 0.45 for CD. The outer shape of the optical pickup, restrictions in optical design due to the increase in the number of optical components for improvements in recording density and signal recording qualities, and restrictions from characteristics of the objective lens actuator limit the choice of an objective lens such that the diameter of the objective lens cannot be too large, and thus its focus distance becomes smaller. Hence, the distance from the objective lens to an optical disc is short and thus, working distance in the focusing direction of the objective lens is short.
In the case where with a single optical pickup, conventional DVD and CD standard discs as well as Blu-ray standard discs are dealt with, a single objective lens preferably deals with optical discs of each standard in terms of configuration. However, with such an optical pickup having a single objective lens mounted, because of the difference in thickness of the cover layer covering the signal layer of optical discs, namely, 0.1 mm for the Blu-ray standard, 0.6 mm for DVD and 1.2 mm for CD, working distance in the focusing direction of the objective lens becomes even shorter.
Optical disc apparatuses which optically reproduce and/or record a signal from and onto optical discs using laser light comprise a focus servo apparatus which moves focus position of the laser light directed to the signal layer of an optical disc from the optical pickup along the focusing direction in a focus search operation, thereby bringing the focus position into a focus servo area.
In order to move focus position of the laser light from the optical pickup along the focusing direction, the optical pickup usually comprises a lens holder having an objective lens mounted and elastically supported to be displaceable along two directions orthogonal to each other including the optical axis direction, and an objective lens driver for driving the lens holder by electromagnetic force. The focus servo apparatus operates the objective lens driver to drive the lens holder, thereby displacing the objective lens along the optical axis direction for achieving the focus search operation (refer to Japanese Patent Application Laid-Open Publication No. 2004-273023).
In the focus search operation of displacing the objective lens along the optical axis direction, the objective lens may collide with the optical disc depending on the working distance of the objective lens. In particular, optical pickups for optical discs of the Blu-ray standard have a greater risk that the objective lens may collide with the optical disc because the working distance of the objective lens is shorter.
Harmful rotational force about a direction orthogonal to the optical axis of the objective lens may occur due to imbalance in the weight of the lens holder, so that the lens holder is tilted and displaced. In this case, a side of the lens holder gets closer to the optical disc, and thus the risk that the lens holder may collide with the optical disc becomes greater.
Hence, conventionally the lens holder is provided with a protection member protruding closer to the optical disc than the other part and opposite the optical disc, which member is made of a material softer than that of the optical disc cover layer, in order to protect the optical disc and the objective lens by the protection member colliding with the optical disc.
However, the protection member may need to be provided as a separate part because of its material and installation position, or appropriate installation position may not be able to be secured. In addition, when colliding, the material from the protection member may stick to the optical disc.
That is, to protect the optical disc and the objective lens with the protection member does not provide an essential solution.
Meanwhile, there is a method of preventing an objective lens from colliding with an optical disc by electrically controlling and restricting the swing distance by which the objective lens is driven along the focusing direction in the focus search operation. However, it is expected that with the method using the electrical control, the lens holder will tilt or that the objective lens cannot be prevented from colliding with the optical disc due to variation in the restricted swing distance range, or the swing distance of the objective lens may not be enough to reach the focus servo area in the focus search operation. Accordingly, it is difficult to decide control settings such as the restricted swing distance range of the objective lens.